Fix handling of version codes in minSdkVersion when signing.

The APK signing tool, build/tools/signapk, needs to know the API Level
of the oldest platform supported by the APK. The APK's minSdkVersion
may reference that using a number (API Level) or a version code. To
handle the version code case, the existing logic was to see if it
matches $PLATFORM_VERSION_CODENAME and then substitute it with
$PLATFORM_SDK_VERSION.

However, some platforms support multiple version codes. To handle this
scenario, this commit changes the logic for computing the min API
Level for signing purposes to treat any non-numeric minSdkVersion as
$PLATFORM_SDK_VERSION.

Bug: 28715556
Change-Id: I292c96e8928b7e2f8d9716ef2be33a23a87764c4