android build: set SOURCE_DATE_EPOCH to cur time for debug builds

Otherwise One would have to git commit every change to be tested on a phone.

contrib/android/Makefile

@@ -4,7 +4,17 @@ PYTHON = python3
 # for reproducible builds
 export LC_ALL             := C
 export TZ                 := UTC
-export SOURCE_DATE_EPOCH  := $(shell git log -1 --pretty=%ct)
+ifndef ELEC_APK_USE_CURRENT_TIME
+    export SOURCE_DATE_EPOCH  := $(shell git log -1 --pretty=%ct)
+else
+    # p4a sets "private_version" based on SOURCE_DATE_EPOCH. "private_version" gets compiled into the apk,
+    # and is used at runtime to decide whether the already extracted project files in the app's datadir need updating.
+    # So, "private_version" needs to be reproducible, but it would be useful during development if it changed
+    # between subsequent builds (otherwise the new code won't be unpacked and used at runtime!).
+    # For this reason, for development purposes, we set SOURCE_DATE_EPOCH here to the current time.
+    # see https://github.com/kivy/python-for-android/blob/develop/pythonforandroid/bootstraps/common/build/build.py#L567-L579
+    export SOURCE_DATE_EPOCH  := $(shell date +%s)
+endif
 export PYTHONHASHSEED     := $(SOURCE_DATE_EPOCH)
 export BUILD_DATE         := $(shell LC_ALL=C TZ=UTC date +'%b %e %Y' -d @$(SOURCE_DATE_EPOCH))
 export BUILD_TIME         := $(shell LC_ALL=C TZ=UTC date +'%H:%M:%S' -d @$(SOURCE_DATE_EPOCH))

contrib/android/make_apk

@@ -71,6 +71,7 @@ else
             -storepass "$P4A_DEBUG_KEYSTORE_PASSWD" \
             -keypass "$P4A_DEBUG_KEYALIAS_PASSWD"
     fi
+    export ELEC_APK_USE_CURRENT_TIME=1
     # only build one apk for debug build, for faster testing iterations
     export APP_ANDROID_ARCH=arm64-v8a
     make apk