am: Allow more hidden apps on devices with lots of RAM

 * If more than 1.5GB is present, allow up to 40 hidden apps.
 * Number is somewhat arbitrary, but was found to work well on D2.
 * Also look at the sys.mem.max_hidden_apps value if given.

Change-Id: I2d3cf6e0dad1bc0b482cc37bf9c61ed2f2b4be51