Map numeric dpi value to density bucket for recovery resources.

When choosing the density for recovery resources, the code used to
handle primary density values only, i.e. mdpi, hdpi, xhdpi, 400dpi,
xxhdpi, 560dpi and xxxhdpi. Unlisted values, such as 500dpi, will be
categorized as xhdpi, but with tiny font size selected for recovery.

This CL improves the handling of numeric dpi values. It allows targets
using numeric values in PRODUCT_AAPT_PREF_CONFIG, and maps that to a
proper density bucket. Targets can still specify the density bucket
directly, which will take priority.

Bug: 111559919
Test: `m -j bootimage` with marlin/sailfish/walleye/taimen respectively.
      Check the chosen density bucket for recovery resources
      (xxxhdpi/xxhdpi/xxhdpi/xxxhdpi).
Test: Set PRODUCT_AAPT_PREF_CONFIG to 279/280/281/500/559/560/640. Check
      the computed bucket and the selected recovery font file.
Change-Id: Ia40fcdec5a2752c08172716bcc622a36a2a83cea