build: Enable custom ccache cache dir for Android

During lunch, check environment variables for ANDROID_CCACHE_DIR.
If it is specified, CCACHE_DIR is set to match. Prevents issues
with multiple versions of ccache writing to the same cache dir.

ANDROID_CCACHE_SIZE should also be specified, allowing for a distinct
cache directory size from the default. Syntax for ANDROID_CCACHE_SIZE
is the same as applied in the command: 'ccache -M$ANDROID_CCACHE_SIZE'

Change-Id: I17497a6f56347850e0fa7b8ebd4de0b5a0b13e55