Support prebuilt apk source selection based on PRODUCT_AAPT_PREF_CONFIG

Two new LOCAL variables are added to support dpi-specific prebuilt apk
selection:
- LOCAL_DPI_VARIANTS: specify a list of dpis the module provides with
  specific prebuilt.
  Example: LOCAL_DPI_VARIANTS := xxhdpi xhdpi hdpi mdpi
- LOCAL_DPI_FILE_STEM: specify the dpi-specific source file name
  pattern.
  Example: LOCAL_DPI_FILE_STEM := MyApp-%.apk
  "%" will be substitued by $(PRODUCT_AAPT_PREF_CONFIG) in the core
  build system.
  If you don't set up LOCAL_DPI_FILE_STEM, the default is
  $(LOCAL_MODULE)_%.apk.

The build system searches $(PRODUCT_AAPT_PREF_CONFIG) in a prebuilt apk
module's $(LOCAL_DPI_VARIANTS). If not found, use whatever
$(LOCAL_SRC_FILES) as the source file; Otherwise use $(LOCAL_DPI_FILE_STEM)
to construct the dpi-specific apk's source file name, and use whatever
directory name of $(LOCAL_SRC_FILES).

Bug: 18388705
Change-Id: I63cae73f1b6f880302142abc476b3ce1fb5500b5